Notch signaling requires endocytosis of both the receptor and its ligand, and endosomal routing of internalized Notch has implications for signal transmission as well as recycling and degradation of unactivated receptors. We have discovered that a classical Drosophila mutant termed big brain (bib) exhibits striking defects in the endosomal accumulation of Notch and other surface-derived proteins. The Bib protein possesses an N-terminal domain that is highly homologous to mammalian aquaporins, which transport water, ions, and other small solutes across biological membranes. Using cell biological and genetic approaches, we have determined that Bib is essential for the maturation of early endosomes and the proper membrane trafficking of Notch in the endosomal pathway. In the absence of Bib function, endosomes are arrested primarily at an early invaginating stage and the formation of acidified lysosomal organelles is impaired. These effects are accompanied by aberrant subcellular trafficking of the intracellular Notch fragment produced by &amp;#947;-secretase cleavage, emphasizing the importance of Notch membrane trafficking for efficient signal transduction. This project has uncovered a novel function for an aquaporin family member in organelle biogenesis within the endosome-lysosome trafficking route, and our initial studies were published recently in Cell (Kanwar and Fortini, 2008). Our future plans include searches for Bib-interacting proteins, and more detailed studies on the relationship of Bib to other endosomal trafficking factors.